PhabricatorAPI: improve code documentation, scope review

Summary:
Code documentation

• Array type is set to 'array' and not 'Array' for @return @var and @param

Scope review

Some methods are now now private: fetchFromAPI, isCached

Test Plan: php artisan phabricator:projectsmap after having applied D165.

Reviewers: dereckson

Differential Revision: http://devcentral.nasqueron.org/D164